The corrosion of the rebar in steel-reinforced concrete structures is a phenomenon of considerable economic importance. J. Tinnea in Materials Performance, 26 (12), 9 (1987), reported that 243,000 bridges under FHWA monitoring were judged to be structurally deficient and in need of repair. In 1986, the Transportation Research Board (TRB) estimated that $20 billion was required to rehabilitate and repair corrosion-induced damage on existing bridge decks nationwide. The repair cost was estimated to be increasing by $500 million annually. In a 1991 report by the Secretary of Transportation to the U.S. Congress on the status of the nation's highways and bridges, the backlog of needed bridge repairs due to structural deficiencies as of the end of 1989 was estimated at $55 to $68 billion. The annual average investment needed for 1990 to 2009 to simply maintain the current structural condition of the nation's bridges was estimated at $2 to $3 billion. To improve conditions and reduce structural deficiencies, an annual average investment of $3 to $6 billion would be needed. The report estimated that 7,000 bridges require rehabilitation or replacement each year.
Concrete is typically a very benign environment for steel because of its mildly alkaline nature. In addition, the concrete layer represents a barrier to external agents which promote corrosion such as oxygen and chloride ions, either during fabrication, or by diffusion from the surroundings. However, when chloride is introduced into concrete, the natural passivity of steel in this environment can be severely compromised. Chloride promotes pitting corrosion, leading to destructive corrosion of the steel, and formation of voluminous, non-adherent iron oxides (rust) which as described below can lead to loss of strength and cracking of the concrete. Chloride is commonly introduced to reinforced concrete through the use of deicing salts or chloride-containing admixtures or by exposure to marine atmospheres.
The damage to reinforced concrete structures is caused principally by permeation of the chloride ions through the concrete to the area surrounding the steel rebar. Because the corrosion products are more voluminous than the base metal, pressure is exerted on the concrete from within, leading to cracking and spalling of the concrete. The corrosion also reduces the effective cross-section and, therefore, the strength of the rebar.
Many different techniques are used in attempts to reduce the corrosion of rebar in concrete, including epoxycoated or galvanized rebar, special low-water concrete mixtures, corrosion inhibitors mixed into the concrete and sealants spread on the finished concrete. Each of these methods requires either additional materials and/or labor costs or long-term capital equipment and maintenance costs, or some compromise in the properties of the concrete (e.g., setting time, ease of pouring, viscosity). Epoxy-coated rebars, special concrete mixtures or inhibitors require that special procedures be followed during construction in order to achieve the optimum benefit of the technique.
A primary object of the present invention is to provide a new and improved method to make concrete-embedded steel reinforcing bars resistant to corrosion during the life of the reinforced concrete structure.
A further object of the present invention is to provide conditions under which the treatment of steel reinforcing bars in wet concrete, cement or mortar can be carried out so as to make the steel surface resistant to corrosion attack.
A still further object is to provide a method for making steel reinforcing bars, embedded in concrete, corrosion resistant, which is inexpensive, safe and requires no long-term maintenance, labor or materials costs.